UNIT 3 GENETICS LESSON #30: TRAITS, GENES, & ALLELES Many things come in many forms. Give me an example of something that comes in many forms. Genes, too, come in many forms.
Main Idea #1 The same gene can have many versions. What are genes? You can think of gene as a piece of DNA that provides a set of instructions to a cell to make a certain protein. Each gene has a locus, ( the address )a specific position on a pair of homologous chromosomes. It tells where the gene is located on the chromosome. An allele (uh-leel) is any of the alternative forms of a gene that may occur at a specific locus.
Main Idea #1 Each parent gives one allele. The two alleles may be the same or they may be different. The term homozygous(hoh-moh-zyguhs) describes two of the same alleles at a specific locus. For example, both might code for white flowers. Heterozygous alleles are different from each other. The term heterozygous(heht-uhr-uh- ZY-guhs) describes two different alleles at a specific locus. For example, one might code for white flowers, the other for purple flowers. Homozygous alleles are identical to each other. Compare & Contrast: Distinguish between the terms allele and locus.
Main Idea #2 Genes influence the development of traits. What was the Human Genome Project? It s goal was to find out the sequence of the 3 billion nucleotide pairs that make up a human s genome. A genome is all of an organism s genetic material. A genotype (JEHN-UH-TYP) typically refers to the genetic makeup of a specific set of genes. For example, the genotype of a pea plant includes both of the genes that code for flower color, even if one of the genes are masked (hidden). A phenotype (FEE-NUH-TYP) typically refers to the physical characteristics, or traits, of an individual organism. For example, a pea plant with purple flowers has a phenotype for purple flowers. The plant might have a hidden gene for white flowers, but that does not matter to its phenotype.
Main Idea #2 If an organism is heterozygous for a trait, which allele will be expressed? A dominant allele is the allele that is expressed when two different alleles or two dominant alleles are present. Upper case letters are used for dominant alleles. A recessive allele is the allele that is expressed only when two copies are present. Lower case letters are used for dominant alleles. That is, if a plant has one allele for purple flowers and one for white flowers, what color will the flowers be? A plant s genotype might be homozygous dominant (TT), heterozygous (Tt), or homozygous recessive (tt).
Main Idea #2 Some alleles are dominant over others, two genotypes produce the dominant phenotype. For example, smooth pods & constricted pods in pea plants are phenotypes. A plant with smooth pods could have a homozygous dominant (SS) or heterozygous (Ss) genotype. What makes one allele dominant over another? It depends on the protein that is, or is not, made. For example, if W directs flower cells to make pigment, the flower may look purple. If w directs the cells not to make pigment, the flower looks white. So W codes for pigment to be present, but w codes for nothing, the absence of pigment. As a result, W has to be dominant. Even if the flower has only one W allele (Ww), that one allele tells its cells to make pigment, and the flower has color. Flower color is only one example.